1. Field of the Invention
This invention relates to a dyeing method employing sulfur dyes or sulfurized vat dyes.
2. Brief Description of the Prior Art
There is known a method of dyeing textile substrates with a sulfur dye or sulfurized vat dye in which an alkaline reducing agent is employed. The method comprises reducing a sulfur dye or sulfurized vat dye with an alkaline reducing agent to obtain a compound (leuco compound) which is readily adsorbed on a textile substrate, immersing a textile substrate in a dye bath containing the leuco compound as dissolved therein and oxidizing the leuco compound in situ, i.e. as adsorbed on the textile substrate, to a water-insoluble sulfur dye or sulfurized vat dye. Among examples of said alkaline reducing agent, there are sodium sulfide, sodium hydrogen sulfide, sodium hydrosulfite-alkali, sodium formaldehyde sulfoxylate-alkali, glucose-alkali, molasses-alkali, thiourea dioxide-alkali and so forth. However, any dyeing procedure using such an alkaline reducing agent has several disadvantages. For example, when the alkaline reducing agent is sodium sulfide or sodium hydrogen sulfide (hereinafter referred to as reducing agent A), the following drawbacks are inevitable. (1) The dye bath containing the reduced dyestuff is readily oxidized by atmospheric oxygen to yield a water-insoluble dyestuff on the bath surface and this dyestuff tends to be deposited on the textile substrate to interfere with level dyeing. (2) Treating the residual or exhausted bath liquor liberates a poisonous gas, i.e. hydrogen sulfide, thus presenting a pollution problem. When the alkaline reducing agent is any of sodium dithionite-alkali, sodium formaldehyde sulfoxylate-alkali, glucose-alkali, molasses-alkali and thiourea dioxide-alkali (hereinafter referred to as reducing agent B), the following drawbacks are encountered. (3) Depending on the dyestuff used, especially in the case of a black dye, it cannot be successfully reduced to the leuco form and, therefore, this method can be utilized only with a dye selected from a limited group of dyes. (4) Even with such dyes, uneven dyeing takes place in many instances, although this disadvantage is somewhat less pronounced than with reducing agent A. Furthermore, irrespective of whether reducing agent A or reducing agent B is employed, the probability of adsorption on textile substrates (dyeing efficiency) is low for certain dyes and in order that the textile substrates are to be dyed deep shades, there must be prepared dye baths containing large amounts of dyestuffs. Of course, this is an uneconomical proposition. By way of illustration, when reducing agents A are employed, dyeing efficiency is low for blue dyes, while reducing agents B provide for only inadequate dyeing efficiency when the dyestuffs belong to the categories of yellow, orange and black.